Thin film triodes and method of forming

ABSTRACT

A thin film structure is described having two metal electrodes separated by a 50 to 500A. thick insulating layer of a material, e.g. aluminum oxide, having a sufficiently large band gap to negate current flow therethrough. At least a portion of one edge of the structure is separated from a control electrode by a semiconductive layer, e.g. vacuum deposited CdS, and tunneling current flow between the metal electrodes through the semiconductive layer is regulated by application of a suitable biasing voltage to the control electrode. In an alternate structure, the control electrode is positioned between and insulated from, the metal electrodes by thin films of a large band gap insulating material. A semiconductive layer overlies one edge of the laminar structure and tunneling current between the exterior electrodes through the semiconductive layer is controlled by a potential applied to the control electrode.

United States Patent [72] Inventor Ivar Giaever Schenectady, N .Y. [21] Appl.No. 829,322 [22] Filed June2,1969 [45] Patented Mar.9,197l [73] Assignee General Electric Company [54] THIN FILM TRIODES AND METHOD OF FORMING 14 Claims, 3 Drawing Figs.

[52] U.S.Cl 317/235, 307/298 [51] Int.Cl ..H01ll3/00 [50] FieldofSeai-ch 317/234, 235,235/8,235/8.1,26l;29/569,576,580

[5 6] References Cited UNITED STATES PATENTS 2,208,455 7/1940 Glaseretal 3l7/235x 2,648,805 8/1953 Spenkeetal. 317/235 3,116,427 12/1963 Giaever 317/235x 3,121,177 2/1964 Davis 317/235x 3,250,967 5/1966 Rose 3,400,456 9/1968 Hanfmann Primary Examiner-James D. Kallam Att0rneys-Paul A. Frank, John F. Ahern, John J. Kissane, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman tioned between and insulated from, the metal electrodes by thin films of a large band gap insulating material. A semiconductive layer overlies one edge of the laminar structure and tunneling current between the exterior electrodes through the semiconductive layer is controlled by a potential applied to the control electrode.

PATEIITEDHAR 9197:;

FIG.

SHEET 1 BF 2 DEPOSIT METALLIC ELECTRODE ATOP INSULATING SUBSTRATE SEQUENTIALLY OVERLAY WITH, LARGE BAND GAP INSULATING FILM AND COUNTER ELECTRODE REMOVE AT LEAST A PORTION'OF ONE EDGE OF LAMINAR STRUCTURE DEPOSIT SEMICON DUCTIVE LAYER ATOP E XPOSED EDGE DEPOSIT CONTROL ELECTRODE ATOP SEMICONDUCTIVE LAYER IBOND LEADS TO ELECTRODES FIG. 2

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l6 I0; I 20 I4 /2 //V/// ////A INVENTOR [VAR G/AEVER,

byarLb/ waw HIS ATTORNEY THEN FTLM TRIODES AND METHOD OF FORMING THE DISCLOSURE This invention relates to thin film tunneling devices and to the method of their fabrication. In a more particular aspect, the invention relates to thin film devices wherein tunneling current flow between the edges of two closely positioned electrodes is controlled by an electrical signal applied to a relatively small band gap material, e.g. a semiconductor, disposed in electrical contact with the edges of the tunneling electrodes. l-ligh mobility tunneling devices in accordance with this invention can be fabricated by vacuum deposition of the semiconductive layer along an exposed edge of the tunneling electrodes with the close proximity of the electrodes effectively producing a single crystalline electrical characteristic in the deposited polycrystalline or amorphous semiconductive layer.

Thin film devices utilizing quantum-mechanical tunneling of elementary charge carriers heretofore have been characterized by a laminar metal-insulator-metal structure wherein tunneling current flows through the insulator spacing the metal electrodes. Tunneling structures of the heretofore mentioned type (both with and without an intermediate film of cadmium sulfide between electrodes) also have been formed atop single crystalline semiconductive substrates to permit electrical signals applied between the metallic electrodes to appear in amplified form at an output terminal bonded to the substrate. In these devices however a large percentage of tunneling current flow is diverted to the control electrode situated atop the semiconductive substrate and high mobility is achieved only with single crystal-line semiconductive substrates.

Another thin film amplifying device utilizes two electrodes deposited on a single crystalline semiconductive substrate which substrate provides a barrier to current flow between electrodes. A thin oxide film serves to space apart and insulate the two slightly overlapping electrodes and, upon application of a suitable potential between the electrodes, the barrier to current flow in the substrate breaks down at the edge of one electrode to emit electrons into the substrate for collection by an electrically biased collector contact to the substrate. Collector current flow in the amplifying device however is by conventional Schottky emission with current control being achieved by constriction of the flow channel at the edges of the electrode atop the substrate. Although some tunneling current may flow between the overlapping portions of the juxtaposed emitter and base electrodes, this tunneling current flow is subject to the deficiencies heretofore described for the metal-oxide-metal-semiconductive substrate laminar structure.

Similarly conventional thin film field effect transistors heretofore have been characterized by a semiconductive layer deposited atop spaced apart metallic source and drain electrodes on an insulating substrate. The portion of the semiconductive layer between the source and drain electrodes is sequentially overlayed with an insulator and gate electrode to permit control of current fiow between the source and drain electrodes by a potential applied to the gate electrode. The spacing between the source and drain electrodes however generally is in the order of microns and current transport of into the semiconductive layer is achieved by Schottky emission. Moreover for good mobility of the charge carriers in the semiconductive layer, the semiconductive layer must be grown epitaxially to offer a single single crystalline orientation for current flow.

It is therefore an object of this invention to provide a high efficiency thin film tunneling device wherein tunneling current is controlled in response to an electrical signal applied to a nontunneling electrode of the structure.

It is also an object of this invention to provide an'improved controlled tunneling device having electrodes of selected metals for producing enhanced controlled tunneling characteristics.

It is a further object of this invention to provide a controlled tunneling device having an amorphous or polycrystalline semiconductive layer forming the current flow channel.

It is a still further object of this invention to provide a simplified method of forming a controlled tunneling device.

These and other objects of this invention generally are achieved by a thin film laminar device characterized by first and second metallic film electrodes spaced apart by an insulating film of a material having a band gap sufficiently large to substantially negate electron fiow between electrodes. An insulating layer having a work function sufficiently small relative to the electrodes to carry essentially all current flow between electrodes bypasses the insulating film while the thin dimension of the large band gap insulating film, e.g. 50-500A; permits current flow in the bypassing insulating layer to be achieved by quantum-mechanical tunneling upon application of a suitable potential to the spaced apart electrodes. A control electrode having a large work function relative to the insulating layer is electrically bonded to the insulating layer and means are provided for controlling the tunneling current flow through the insulating layer by the application of a suitable electrical signal to the control electrode to alter the flow channel between the tunneling electrodes.

Because of the extremely thindimension of the insulating film between the tunneling electrodes, the insulating layer interconnecting the electrodes acts as a single crystalline structure to charge carrier flow between-electrodes notwithstanding an amorphous or polycrystalline structure in the insulating layer. Thus,- the insulatinglayer can be formed by conventional vapor deposition of a semiconductor atop an exposed edge of the laminar structure in counterdistinction to conventional solid state devices requiring epitaxial growth of the semiconductive layer for high charge carrier mobility.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, together with further objects and advantages thereof may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a flow chart depicting in block diagram form the method of this invention,

FIG. 2 is a pictorial illustration of 'thefabrication of a thin film tunneling device in accordance with this invention, and

FIG. 3 is an isometric view of an alternate tunneling device constructed in accordance with this invention.

The method of forming a tunneling device in accordance with this invention is illustrated in FIGS. 1 and 2 and initially comprises the vacuum deposition of metallic electrode 10 upon an insulating substrate 12 previously prepared in conventional fashion for deposition by being washed in detergent. rinsed with de-ionized water and dried. Desirably electrode 10 is a metal having a small work function relative to a subsequently to be deposited semiconductive layer (as will be more fully explained hereinafter) and suitably may be a metal such as indium, tin or aluminum'deposited to a thickness of. for example, 1,000A. Although any conventional technique for forming electrode l0 can be employed. vacuum deposition of the chosen metal at a pressure between l X 10-to l X 10- torr generally is suitable.

After the deposition of electrode l0atop substrate 12, an insulating film l4 and-a counterelectrode 16 are sequentially vacuum deposited thereon to form laminar structure 18. lnsulating film l4 desirably is of a material having a large band gap to negate current flow between electrodes 10 and l6 through the insulating film. Among the more suitable insulating films for this purpose are high purity aluminum oxide, silicon monoxide or silicon dioxide with vacuum evaporation of spectroscopic grade pellets of the chosen insulator at pressures of approximately l X l0- torr generally being suitable for deposition of the insulating film atop electrode 10. Desirably, the chosen insulating film is deposited to a thickness in excess of 50A. to inhibit tunneling current flow through insulating film 14; during operation while the maximum thickness of the insulating film should be below 500A. to permit tunneling of charge carriers through a subsequently to be deposited relatively smaller band gap material, e.g. a semiconductor, electrically bypassing the insulating film.

Counter electrode 16 normally is a low work function metal identical to the metal employed to deposit electrode 10, e.g. indium, tin, aluminum, etc. and suitably is deposited to a thickness of approximately 1,000A. Any electrode thickness however can be employed, if desired, provided the electrode is electrically continuous and of sufficient thickness to carry the s desired tunneling current flow therethrough.

After formation of laminar structure 18, at least a portion of one edge of the structure is mechanically removed, e.g. cut away with a knife edge, to expose each layer forming the laminar structure whereupon a layer 20 of, for example, cadmium sulfide is deposited to a thickness of approximately 2 mils atop the exposed edge of the laminar structure. In general, layer 20 can be any nonmetal characterized by a small work function with metal electrodes 10 and 16 relative to the work function between the metal electrodes and insulating film 14 and thus would include high conductivity insulators, such as the semiconductors cadmium sulfide, lead sulfide, germanium, silicon, etc., as well as more conventional insulators such as zinc oxide or lead oxide doped to a resistivity between 10- and l ohm-cm. For example, for an insulating film 14 having a work function in the range of 3 to 4 ev. relative to electrodes 10 and 16, layer 20 may be characterized by a work function of approximately one-half ev. relative to the electrodes to maximize current flow between electrodes in layer 20. Desirably, the'work function between layer 14 and metallic electrodes 10 and 16 should be at least approximately twice the work function between the metallic electrodes and layer 20 to assure at least 70 percent of the current flow between electrodes passes between the edges of the electrodes by way of layer 20.

Because of the close proximity between the electrodes forming laminar structure 18, e.g. insulating film l4 spacing the electrodes preferably is of a thickness between 50A. and 200A. although insulator film thicknesses as high as 500A. may be employed with semiconductor materials such as cadmium sulfide and lead sulfide having a free carrier charge concentration of 10"carriers/cm; a high mobility is observed by tunneling current flowing through layer 20 notwithstanding an amorphous or polycrystalline structure in the layer. Thus epitaxial growth of a semiconductor is not required and layer 20 can be conveniently formed by evaporation of cadmium sulfide in a vacuum of approximately I X 10-to l X lO-torr. To assure good electrical contact between the deposited semiconductor and the metallic electrodes forming laminar structure 18, the evaporating crucible (not shown) desirably is placed to the side of the laminar structure adjacent the removed edge rather than directly below the laminar structure. Thus, ideally a straight line drawn between the center of laminar structure 18 and the center of the crucible evaporating layer 20 forms an angle between 60 and 12 with the planes of electrodes 10 and 16.

Subsequent to the deposition of layer 20, a control electrode 22 is deposited atop the face of layer 20 remote from laminar structure 18' to regulate the tunneling cur-rent flow through layer 20. Desirably control electrode 22 is a metal, e.g. platinum, gold, etc., having a relatively high work function with layer 20 to inhibit electron flow from the control electrode into the juxtaposed semiconductor layer. To assure good coverage of the laminar structure edge, the control electrode source is located in the vacuum deposition chamber at a location closely adjacent, or identical to, the location of layer 20 source and evaporation is suitably conducted at a pressure of X -torr. After deposition of the controlelectrode, leads 24, 26 and 28 are electrically bonded to electrodes l0, l6 and 22, respectively, by suitable techniques, e.g. soldering or thermal compression bonding, to permit the application of biasing voltages to the structure.

In operation, a voltage source 30 of approximately 0.1 volt is connected between electrode 10 and counter electrode 16 to produce a tunneling current fiow 32 from the edges of the electrodes through layer 20 with current flow through insulating layer 14 being inhibited by the wide band gap of the insulating layer. When a variable control voltage source 34 of approximately 1.0 0.1 volts is applied between control electrode 22 and electrode 10, equipotential lines of force 36 extend into layer 20 to constrict or enhance the tunneling current flow therethrough in proportion to the magnitude and polarity of the applied control voltage.

An alternate tunneling thin film structure 38 in accordance with this invention is depicted in FIG. 3 and generally comprises a laminar structure having the control electrode 40 centrally disposed between tunneling electrodes 42 and 44. Structure 38 is similar to laminar structure 18 of FIG. 1 except for the positioning of control electrode 40 and is characterized by two metallic tunneling electrodes 42 and 44 having a low work function, e.g. one-half ev. relative to a semiconductive layer 46 of, for example, cadmium sulfide disposed in electrical contact with'each layer of the laminar structure along an exposed edge thereof. Control electrode 40 is of a material, eg platinum or gold, having a large, e.g. l.5 ev. or larger, work function for charge carriers to overcome to be injected into semiconductor layer 46 and is electrically isolated from tunneling electrode 42 and 44 by insulating films 48 and 50, respectively, having a sufficiently large band gap, e.g. 3 ev. to substantially negate current flow from the tunneling electrodes to the control electrode.

Structurally, electrodes 42 and 44 may be any thickness, e.g. approximately 1,000A; while the maximum total dimension of control electrode 40 and insulating films 48 and 50 must be less than the total dimension permitting tunneling current to flow between electrode 42 and 44 through semiconductor layer 46. Thus, for a cadmium sulfide semiconductor having a maximum tunneling current dimension of approximately 500A., control electrode 40 ideally is approximately 200A. thick with insulating layers 48 and 50 each being approximately 10OA. in thickness.

Because of the large band gap of the insulating films 48 and 50, charge carrier flow between electrodes 42 and 44 through the insulating films generally is inhibited. To further limit charge carrier flow through theinsulating films, however, the width W of the electrodes should be minimized, e.g. to approximately l0 microns, thereby reducing the capacitance between electrodes 42 and 44 and increasing the switching speed of the tunneling structure.

Structure 38 can be fabricated by the sequential vapor deposition of electrode 42, insulating film 48, control electrode 40, insulating film 50 and electrode 44 from crucibles positioned directly below the insulating substrate 52 accepting the deposition of the layers thereon. The edge of the structure then is mechanically removed, e.g. by a knife edge or by removal of a second substrate (not shown) masking a portion of substrate 52, to expose each of the laminar layers and a semiconductive material, such as cadmium sulfide, is evaporated from a crucible positioned aside the exposed edge of the laminar structure to assure good electrical contact between deposited semiconductive layer 46 and each of the films forming structure 38. Although vapor deposited cadmium sulfide characteristically possesses a polycrystalline structure, other materials such as germanium having an amorphous structure when deposited upon an unheated substrate also can be employed in forming tunneling structures in accordance with this invention with the close proximity of electrodes 42 and 44 resulting from the thin film laminar configuration of structure 38 producing a high mobility in charge carrier flow through semiconductive layer 46 notwithstanding the polycrystalline or amorphous nature of the layer. lt will be appreciated that the insulating films separating the electrodes also can be formed by oxidation of an electrode surface when the electrode is of a metal, e.g. aluminum, which is readily oxidizable.

The operation of tunneling device 38 is similar to that shown in FIG. 1 with an applied voltage of 0.1 to 1 volt from source 54 connected between electrodes 42 and 44 producing a tunneling current fiow 56 through amorphous or polycrystalline semiconductive layer 46. A time varying control voltage source 58 of 0.1 to 1 volt then is applied between control electrode 40 and electrode 44 to form equipotential lines of force 60 within the semiconductive layer regulating the flow of tunneling current therein with current flow from the control electrode into the semiconductive layer being inhibited by the large work function between the control electrode and the semiconductive layer.

1 claim:

1. A thin film device comprising first and second electrodes, an insulating film disposed between said electrodes, said insulating film having a band gap sufficiently large to substantially inhibit current flow between said electrodes through said insulating film, an insulating layer bypassing said insulating film and interconnecting said first and second electrodes, said insulating layer having a work function sufficiently small relative to the electrodes to carry essentially all current flow between said electrodes, the flow channel between said electrodes in said insulating layer being sufficiently small to permit current fiow due to quantum-mechanical tunneling in response to a suitable potential applied between electrodes, means for applying said suitable potential between said electrodes, a control electrode electrically bonded to said insulating layer and means for applying an electrical signal to said control electrode to modulate the flow of tunneling current between said first and second electrodes.

2. A thin film device according to claim 1 wherein said first and second electrodes and said insulating film form a laminar structure, at least one edge of said structure being exposed along a plane angularly disposed relative to the planes of said electrodes to bare the component layers of said laminar structure, and said insulating layer is deposited atop said exposed edge to interconnect said first and second electrodes.

3. A thin film device according to claim 2 wherein said insulating layer has an amorphous structure.

4. A thin film device according to claim 2 wherein said insulating film is between 50 and 500A. in thickness, and said control electrode is a material having a large work function relative to said insulating layer to substantially negate current flow therebetween, said control electrode being disposed along the face of said insulating layer remote from said exposed edge of said laminar structure.

5. A thin film device according to claim 4 wherein said first and second electrodes are low work function metals selected from the group consisting of indium, tin, and aluminum, said insulating film is an oxide of a metal selected from the group consisting of aluminum and silicon, said insulating layer is a semiconductor selected from the group consisting of cadmium sulfide, lead sulfide, germanium and silicon and said control electrode is a metal selected from the group consisting of gold and platinum.

6. A thin film device according to claim 2 wherein said control electrode is disposed intermediate said laminar structure at a location insulated from said first and second electrodes by said insulating film, said control electrode having a work function with said insulating layer substantially in excess of the work function between said insulating layer and said first and second electrodes.

7. A thin film device according to claim 6 wherein said electrodes are metals selected from the group consisting-of indium; tin and aluminum, said insulating film is an oxide of a metal selected from the group consisting of aluminum and silicon, said insulating layer is a semiconductor selected from the group consisting of cadmium sulfide, lead sulfide, germanium, and silicon and said control electrode is a metal selected from the group consisting of gold and platinum.

8. A thin film device comprising first and second metallic electrodes spaced apart by a 50 to 500A. thick insulating film, said insulating film having a band gap sufficiently large to substantially inhibit charge flow between said electrodes, a polycrystalline semiconductive material bypassing said insulating film and interconnecting said electrodes, means for applying an electrical potential across said electrodes to produce a quantum-mechanical tunneling current flow between said electrodes through said polycrystalline semiconductive material, and means for applying a control signal to said polycrystalline material to regulate the flow of tunneling current therethrough.

9. A thin film device according to claim 8 wherein the work function between said electrodes and said polycrystalline material is at least 1.5 ev. smallerthan the work function between said electrodes and said insulating film.

10. A thin film device according to claim 8 wherein said insulating film is aluminum oxide in a thickness between 50 and 300A. and said polycrystalline material is a sulfide of a metal selected from the group consisting of cadmium and lead.

11. A method of forming a thin film device comprising vacuum depositing a metallic electrode upon a dielectric substrate, forming a 50-50OA. thick insulating film atop said metallic electrode, vacuum depositing a metallic counter electrode atop said insulating film to form a metal-insulator-metal laminar structure, exposing at least one edge of said laminar structure along a plane angularly disposed relative to the planes of said electrodes, vacuum depositing a layer of semiconductive material atop said exposed edge, said semiconductive material having a small work function relative to the electrodes of said laminar structure to permit quantummechanical tunneling of charge carriers between said electrodes through said semiconductive material and depositing a metal electrode atop said semiconductive layer.

12. A method of forming a thin film device according to claim 11 wherein the d edge of said laminar structure is exposed by positioning a second substrate atop a portion of said dielectric substrate prior to deposition of said laminar structure and removing said second substrate after deposition of said laminar structure to expose the edge of said structure.

13. A method of forming a thin filmdevice according to claim 12 wherein said semiconductive layer is vacuum deposited from a source disposed at a location aside the exposed edge of said laminar structure.

14. A method of forming a thin film device according to claim 11 wherein said edge is exposed by dissecting said laminar structure along a plane normal to the plane of said electrodes forming the laminar structure. 

2. A thin film device according to claim 1 wherein said first and second electrodes and said insulating film form a laminar structure, at least one edge of said structure being exposed along a plane angularly disposed relative to the planes of said electrodes to bare the component layers of said laminar structure, and said insulating layer is deposited atop said exposed edge to interconnect said first and second electrodes.
 3. A thin film device according to claim 2 wherein said insulating layer has an amorphous structure.
 4. A thin film device according to claim 2 wherein said insulating film is between 50 and 500A. in thickness, and said control electrode is a material having a large work function relative to said insulating layer to substantially negate current flow therebetween, said control electrode being disposed along the face of said insulating layer remote from said exposed edge of said laminar structure.
 5. A thin film device according to claim 4 wherein said first and second electrodes are low work function metals selected from the group consisting of indium, tin, and aluminum, said insulating film is an oxide of a metal selected from the group consisting of aluminum and silicon, said insulating layer is a semiconductor selected from the group consisting of cadmium sulfide, lead sulfide, germanium and silicon and said control electrode is a metal selected from the group consisting of gold and platinum.
 6. A thin film device according to claim 2 wherein said control electrode is disposed intermediate said laminar structure at a location insulated from said first and second electrodes by said insulating film, said control electrode having a work function with said insulating layer substantially in excess of the work function between said insulating layer and said first and second electrodes.
 7. A thin film device according to claim 6 wherein said electrodes are metals selected from the group consisting of indium, tin and aluminum, said insulating film is an oxide of a metal selected from the group consisting of aluminum and silicon, said insulating layer is a semiconductor selected from the group consisting of cadmium sulfide, lead sulfide, germanium, and silicon and said control electrode is a metal selected from the group consisting of gold and platinum.
 8. A thin film device comprising first and second metallic electrodes spaced apart by a 50 to 500A. thick insulating film, said insulating film having a band gap sufficiently large to substantially inhibit charge flow between said electrodes, a polycrystalline semiconductive material bypassing said insulating film and interconnecting said electrodes, means for applying an electrical potential across said electrodes to produce a quantum-mechanical tunneling current flow between said electrodes through said polycrystalline semiconductive material, and means for applying a control signal to said polycrystalline material to regulate the flow of tunneling current therethrough.
 9. A thin film device according to claim 8 wherein the work function between said electrodes and said polycrystalline material is at least 1.5 ev. smaller than the work function between said electrodes and said insulating film.
 10. A thin film device according to claim 8 wherein said insulating film is aluminum oxide in a thickness between 50 and 300A. and said polycrystalline material is a sulfide of a metal selected from the group consisting of cadmium and lead.
 11. A method of forming A thin film device comprising vacuum depositing a metallic electrode upon a dielectric substrate, forming a 50-500A. thick insulating film atop said metallic electrode, vacuum depositing a metallic counter electrode atop said insulating film to form a metal-insulator-metal laminar structure, exposing at least one edge of said laminar structure along a plane angularly disposed relative to the planes of said electrodes, vacuum depositing a layer of semiconductive material atop said exposed edge, said semiconductive material having a small work function relative to the electrodes of said laminar structure to permit quantum-mechanical tunneling of charge carriers between said electrodes through said semiconductive material and depositing a metal electrode atop said semiconductive layer.
 12. A method of forming a thin film device according to claim 11 wherein the d edge of said laminar structure is exposed by positioning a second substrate atop a portion of said dielectric substrate prior to deposition of said laminar structure and removing said second substrate after deposition of said laminar structure to expose the edge of said structure.
 13. A method of forming a thin film device according to claim 12 wherein said semiconductive layer is vacuum deposited from a source disposed at a location aside the exposed edge of said laminar structure.
 14. A method of forming a thin film device according to claim 11 wherein said edge is exposed by dissecting said laminar structure along a plane normal to the plane of said electrodes forming the laminar structure. 